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Rudist reefs developed in a variety of paleogeographic settings in the shallow Cretaceous seas of the western Gulf of Mexico region. Some reefs were built along the shelf edge, others grew on the inner shelf relatively nearshore, and others were built on the broad, open shelf. Although these reefs vary in morphology, size, biologic and sedimentologic characteristics, and relation to associated facies, probably none grew in waters deeper than a few tens of feet. The most persistent feature of these reefs is the abundant evidence of repeated subaerial exposure, early diagenesis, and erosion during deposition.
Several exceptionally well-exposed reefs in the Albian and Cenomanian of Texas and northeastern Mexico provide models for interpreting the geologic history of rudist reefs elsewhere in the region. The core of each of these reefs is made of lenses, wedges, and layers of rudist-rich carbonate mudstone in which the rudists are preserved in mutually supporting, original-growth positions alternating with lenses, wedges, and layers of carbonate grainstone composed principally of rudist shell fragments. Laterally contiguous seaward deposits are carbonate grainstone, and backreef deposits are chiefly miliolid-rich carbonate mudstone. Peripheral grainstones originated as forereef debris or storm washover deposits during reef-growth stages, and as beach deposits during island stages. Backreef c rbonate mudstone was deposited as tabular units in quiet-water lagoons, and locally small mounds of diagenetically fragmented rudists characterize the environment.
Truncation surfaces, weathered zones, layers of caliche, bored surfaces, zones of vadose sediment and cement, and other subaerially formed or related features and materials extend through the reef cores and in some places into the adjacent deposits as evidence of repeated exposure and resubmergence. In some reefs as many as 15 exposure periods are recorded vertically within 50 ft of reef core. Many exposure surfaces associated with these reefs are confined to the area of the
reef core, others extend into the laterally contiguous deposits and a few are regional in extent.
Typically, rudist reefs in this region record a history of short periods of organically controlled growth in shallow water alternating with brief periods of subaerial exposure and early diagenesis. These processes result in the formation of laterally restricted but internally complex limestone bodies 10-2,000 ft thick and a few hundreds of square feet to tens of square miles in areal extent.
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